Separation techniques in which semipermeable membrances are used are already known. For example, in one process known as inverse osmosis, a liquid solution is passed through a semipermeable membrane under a pressure higher than the osmotic pressure of this solution in order to separate at least one substance dissolved in this solution. This process can be used for example for the desalination of sea water.
In another process known as ultrafiltration, ultrafiltration membranes are used. These ultrafiltration membranes are generally made of synthetic or natural polymeric substances, especially cellulose derivatives, and have pore diameters of, for example, from 50 to 100,000 Angstroms. By applying ultrafiltration, it is possible to separate from a liquid medium all the substances which are dissolved or dispersed in it in the form of molecules or agglomerates of molecules with dimensions greater than those of the pores of the ultrafiltration membrane used. In view of the pore sizes of the filtering membranes which can be used for carrying out this particular process, the molecular weight of the substances which can be separated from a liquid medium by this process is at least equal to approximately 5000. Accordingly, the field of application of this process extends for example to the separation of substances of biological origin which have a high molecular weight, for example proteins and enzymes.
In certain fields, inverse osmosis, ultrafiltration and known separation techniques of the same kind have numerous technical and economic advantages over other conventional separation techniques such as distillation, solvent extraction, crystallisation, etc., notably by virtue of the fact that these processes, which use semipermeable membranes, do not involve any change of phase either in regard to the substances to be separated or in regard to the solvents.
However, these processes do have certain limitations, notably in the case of inverse osmosis, due to the non-existence of semipermeable membranes with the necessary characteristics for use on an industrial scale in a non-aqueous medium and, in the case of ultrafiltration, due to the fact that it is not possible by this technique selectively to separate substances with a molecular weight of below about 5000 on an industrial scale.
In addition, it is not possible by inverse osmosis to separate substances with low molecular masses, such as simple or complex metal ions, from substances of high molecular weight, such as proteins, synthetic resins in solution, etc.
In addition, it has already been proposed selectively to separate substances with a molecular weight of less than 5000, for example complex metal ions, from a liquid medium containing these substances in solution, by fixing the molecules of these substances to macromolecular materials in solid form, notably to ion-exchange resins. This process has the advantage of extreme effectiveness in regard to the degree of separation, but unfortunately is accompanied by a low yield from the point of view of the ratio between the mass of macromolecular substance used and that fraction of this substance which performs an active function in separation. In addition, this process is not suitable for continuous working, which precludes its use on a commercial scale.
It is an object of the present invention to overcome the foregoing disadvantages.